1. Field
This invention relates to dispersion strengthening of metals. It is specifically directed to the dispersion strengthening of aluminum alloys, and provides a family of such alloys capable of withstanding welding temperatures.
2. State of the Art
Dispersion strengthened metals and methods for enhancing various properties of metals through the dispersion of refractory particles in a metal or alloy are well known. Such metals and processes are disclosed, for example, in U.S. Pat. Nos. 3,028,234 (Alexander, et al.); 3,290,144 (Iler, et al.); and 3,468,658 (Herald, et al.); the disclosures of which are incorporated by reference.
Alexander, et al. is directed to a general method for mixing a powdered solid dispersion of refractory metal oxide particles in an inactive metal with a molten mass of metal to be hardened (notably nickel). Alexander, et al. suggest (in Example 1) that a copper-alumina powder may be added to a molten aluminum alloy. In practice, however, when the procedures of Example 1 are followed, the copper-aluminum powder does not dissolve in the aluminum alloy and thus does not produce a satisfactory dispersion hardened aluminum alloy. Alexander, et al. also teach protecting the copper-aluminum powder in an inert atmosphere to prevent oxidation of the copper prior to adding it to molten aluminum. Alexander, et al. also suggest sintering the powder prior to its introduction to the melt.
Iler, et al. disclose a mechanical method for producing dispersion hardened copper. The method includes the production of a dense billet composed of copper powder with alumina particles dispersed therein. The copper powder is obtained by reducing a copper compound, and is protected by an inert atmosphere to avoid reoxidation prior to being pressed into the dense billet.
Herald, et al. suggest adding a dispersoid such as aluminum oxide to metals in a molten state. Agglomeration is avoided by wetting the dispersoid with the metal to be hardened. "Wetting" is achieved by saturating the metal with the anion of the dispersoid while the dispersoid is being mixed with the molten metal.
SAP (sintered aluminum powder) metal is an example of an oxide metal dispersion hardened aluminum alloy which is known to have a service temperature as much as 200.degree. C. higher than typical aluminum alloys. SAP is produced by mechanical working methods. While it has excellent properties, those properties are permanently destroyed at temperatures approaching welding temperatures.
Other U.S. Patents reflecting the state of the art include Badia et al, U.S. Pat. No. 3,600,163 which teaches the dispersion of graphite in molten aluminum, employing a wetting process. The graphite particles are preferably 40 microns in average cross section size, but graphite particles as small as 20 microns reportedly have given excellent results.
Imich, U.S. Pat. No. 2,793,949 teaches wetting particles of ceramic materials such as emery, corundum, burned alumina, flint, quartz and others into various molten metals. Imich produces composite materials which generally contain 5 to 50 volume percent of the ceramic material. Particle sizes for the ceramic material range from 0.5 microns (Example 11), up to 30 mm in Example 6.